This invention relates to an improvement in a process for refining a ferrous melt by blowing oxygen into the melt from above the melt surface, commonly called the "basic oxygen process". More specifically, this invention relates to a method for preventing or minimizing the overflow of material from the mouth of the vessel which tends to occur during conventional practice of the basic oxygen process.
Oxygen is used to decarburize the melt by reacting it with the carbon contained therein to form CO, which escapes from the vessel as a gas. Typically, the unrefined ferrous melt also contains silicon and other oxidizable elements such as manganese and phosphorus, the oxides of which form liquids or solids which form a separate slag phase. Lime and other materials such as dolomitic lime are added into the vessel to form a basic slag.
It is well known to those skilled in the art that refining is most efficient if what is referred to in the art as an "emulsion" is formed above the melt during the oxygen blow. The emulsion is a foam-like substance comprising a complex mixture of liquid oxides, gas bubbles (primarily CO), solid oxide particles, and droplets of liquid metal. The volume of the emulsion is ideally several times that of the melt; see FIG. 1.
A problem in the basic oxygen process is that the volume of the emulsion is difficult to control. Frequently, the emulsion becomes so large that it slops, that is, it fills the head space of the vessel and overflows from the mouth of the vessel, causing loss of valuable metal and production time, and necessitating time-consuming clean-up.
Prior methods of controlling slopping include the following steps or various combinations thereof:
(1) decreasing the oxygen flow; see for example, Stravinskas et al, "Influence of Operating Variables on BOF Yield", I & SM, May 1978, pp. 33-37;
(2) increasing the oxygen flow; see for example, Zarvin et al, "Some Features of Injection in the Melting of Steel in 350-Ton Basic Oxygen Furnaces", Steel in the USSR, December 1976, Vol. 6 pp. 659-662;
(3) lowering the lance position; see for example, Shakirov et al, "The Mechanism of the Foaming of Basic Oxygen Furnace Slag," Steel in the USSR, June 1976, Vol. 6;
(4) raising the lance position; see for example, Chernyatevich et al, "Mechanism of the Formation of Ejections and Spatter from Basic Oxygen Furnaces", Steel in the USSR, October 1976, Vol. 6, pp. 544-547;
(5) changing the lance nozzle design; see for example, Baptizmanskii et al, "Causes of Ejections and of Lancing Conditions in Basic Oxygen Furnace", Stal, April 1967, pp. 309-312; and
(6) modifications to the amount, ingredients, and timing of flux addition; see for example, Chernyatevich et al, supra. Unfortunately, none of the above methods are very reliable, some are complicated, and some require production delay.